After completion of several of our objectives in FY13, this project has taken a new direction. We are defining the minimal requirements for the activation of NK cell proliferation, using approaches that have been successful in previous projects. The interleukin 15 (IL-15) is essential for NK cell survival and proliferation. Unlike other cytokines, which are soluble, IL-15 is bound to the alpha chain of the IL-15 receptor (IL-15R). The IL-15Ralpha-IL-15 complex is not expressed on NK cells but on other cells, which trans-present IL-15 to the beta and gamma chains of the IL-15R expressed on NK cells. The IL-15Rbeta/gamma complex then transmits signals to the NK cell for survival, proliferation, and transcription of a specific set of genes. Trans-presentation of IL-15 in the context of an immunological synapse opens the possibility that signaling for NK cell proliferation may be subject to regulation by other receptor-ligand interactions. Using insect cells transfected with IL-15Ralpha, either alone or in combination with ligands for other NK receptors, we have shown that IL-15 trans-presentation occurs independently of adhesion through integrin, and of signaling by other activation receptors. The IL-15Ralpha chain was also expressed on a human cell line in combination with HLA class I molecules that serve as ligands for inhibitory receptors. This sytem mimics IL-15 trans-presentation by human cells, which occurs in the context of HLA class I molecules. We have shown that the proliferation of NK cells induced by IL-15 trans-presentation is modulated by the simultaneous engagement of inhibitory receptors. The inhibition occurs at the level of the signals that promote proliferation, but that the Stat5-dependent signals for transcription are not affected.